My guess is a maybe a student gun number from a school house, like as in “rifle #52” (out of…well at least 52
)but the number isn’t in a local that I’ve seen durn my time at SOTIC/SFSC. and the location is kinda weird, like you couldn’t likely read it from the rack but that orientation is perfect for identification of the rifle from the firing line BUT seems like it might wear off quickly through. 
It may possibly fit the 3 digit team numbering convention of SF, as in “0 = 10th SFG(A) 5 = 2nd BN/B Co. And 2 = 2nd ODA. It looks about right for the mid-2000’s which is when the change over to the 4 digit convention occurred. SF held on to the 24s all the way end but it is totally possible that a M24 could have been marked in such a way in the mid-2000’s and survived updated alterations by some junior Bravo will a paint pen. The SR25 was gaining a lot of favor in Iraq for DA support, so sidelining the M24 was understandable for that period.
But honestly 3 digits is pretty vague unless someone has some personal insight.
